Public or community sewage treatment is not available to all houses and businesses. As a result, many houses and businesses must utilize an on-site septic system for sewage treatment and disposal. A conventional on-site septic system includes a septic tank which receives domestic sewage from the home or business, and a soil absorption field or drain field where sewage flows from the septic tank and where treatment of the sewage takes place due to the presence of microbes which act on the sewage to metabolize the carbon and nutrients. Sink waste, toilets, shower/bath, and laundry wash water are the usual contributions to domestic sewage. The septic tank generally functions to separate the solid matter and aqueous fraction into three distinct layers or phases within the tank due to differing densities, settling velocities and detention time. These phases include a solids phase, an aqueous phase, and an oil phase. Generally, it is the middle, aqueous phase that is intended to flow into the soil absorption field.
Conventional septic systems can be quite effective for treating sewage given ideal site conditions. If, however, site conditions and/or temperatures are less than favorable, there exists the potential for inadequate sewage treatment which may result in contamination of ground water and surface water. Even if site and temperature conditions are desirable, sewage from conventional septic systems may leach through the soil and contaminate ground water in areas with particularly high water tables. Accordingly, it would be desirable to provide a sewage treatment system which can treat sewage even under adverse site conditions to provide effluent sufficiently free of contaminants which could enter into ground water and contaminate the ground water supply.
One compound present in the waste water stream that causes particular concern is the nitrate ion, (NO.sub.3.sup.-). Nitrate is often used as an indicator of waste water contamination and is listed as a contaminant that renders water unfit for human consumption if its concentration in a water supply exceeds 10 parts per million. Because nitrate ions are highly soluble in water and are negatively charged, they have a tendency to be repelled by negatively charged soil particles and are easily carried away from soil with water. Accordingly, it would be desirable to provide a system which reduces nitrate in waste water.
Also of concern is the levels of biological oxygen demand (BOD) in the sewage from conventional septic systems. One technique to decrease the level of BOD in septic tank sewage is to process the sewage through a sand filter prior to allowing it to enter the drain field. Such a system significantly reduces the BOD. Nitrogen, however, in its various forms, will not be completely removed by a sand filter, and may ultimately enter the ground water. Additionally, a conventional sand filter may become clogged after a relatively short period of time, requiring replacement of the filter media.
Recirculating sand filters and intermittent sand filters for septic systems are sometimes used where drainage fields are inadequate or not practical. In these systems, sewage flows to a septic tank which provides initial treatment. The septic effluent is discharged into a recirculating pump tank which is provided with a pump controlled by a timer to periodically dose a recirculating sand filter. After the effluent is distributed over the sand filter, the filtrate is collected and directed back through the recirculating pump where a recirculation valve either returns it to the pump tank or discharges the effluent for final treatment and disposal. In an intermittent sand filter the recycle ratio is 1:1, i.e., the waste water makes only one pass through the filter. Unfortunately, sand filters can be very expensive and are generally not effective at reducing nitrate concentration. Accordingly, it would be desirable to provide a system which reduces nitrate concentration in a physically smaller and less costly process unit.
Attached growth and suspended growth systems have been developed to reduce nitrate concentration. In attached growth systems, coarse granular media provides a large surface area for microbial growth within a nitrification unit. Suspended growth systems are commonly referred to as sequencing batch reactors or aerobic package plants. Generally, these systems function by accepting an entire daily flow of domestic sewage into one compartmentalized tank. In a general scheme of operation, the tank is filled with a daily supply of sewage. A mechanical air compressor and/or effluent pump forces air into the tank thereby creating an aerobic and highly agitated environment. At a certain point, the compressor or pump shuts down and the liquid is allowed to become quiescent. The liquid is then disposed to a drain field. These systems are disadvantageous because they are costly and complicated, and because they do not remove an adequate amount of nitrate without requiring the addition of a carbon source.
The Ruck system was designed to reduce nitrogen concentration in waste water by providing both an aerobic and anoxic step. These steps are combined within a single, separate tank as described in U.S. Pat. No. 4,465,594. This system requires splitting waste water flow into two streams, a black water stream and a grey water stream. If the waste water stream is not split, chemical addition for a source of carbon denitrification is needed. Separate septic tanks are used for the black water stream and the grey water stream. The nitrification unit uses "in-drains" which add to the overall complexity of the system. Air is supplied only passively through exposed vents, and no protection against cold air intrusion into the system is provided. In view of this complexity, the Ruck system is fairly expensive.
Accordingly, it would be desirable to provide a system for removing nitrates and reducing biological oxygen demand to acceptable levels using a system which is easy to install, requires minimal maintenance, and which does not require the addition of chemicals. In particular, it is desirable to provide a system which exploits the naturally occurring conditions in the treatment of sewage.